KImg: Polish documentation

[MisRob]

Summary

Even though the documentation for KImg should exist, it'll be put together gradually (and presumably by more devs). This issue is to polish it to ensure consistency and good organization and wording.

Epic

• KImg Component #368

Acceptance criteria

• Contains a summary, examples, and guidelines that are created in cooperation with the design team
• All props are documented
• All slots are documented
• All events are documented

Linked issues

• Blocked by
  • KImg: Implement the basic component #369
  • KImg: Add scaling and placeholder logic #370
  • KImg: Add 'appearanceOverrides' #371

[MisRob]

@jtamiace comment:

i think one thing we can add for design/UX guidelines is an example of how these props and settings are currently applied and why. using thumbnails as the example since it’s the most common use now - why we use the current scaling/aspect ratio settings for it, and the UX considerations that affect those decisions

[MisRob]

Closed by #538